The invention is in the field of semiconductor devices in general and integrated circuit semiconductor devices in particular, and relates specifically to field inversion control for such devices.
In MOS integrated circuits, each individual MOS transistor is surrounded by a thick oxide area formed over the same surface of the substrate. This thick oxide area is called the field oxide, or simply the field. The electrical leads interconnecting the individual MOS transistors pass over this field oxide. If the voltage level of such interconnects with respect to the substrate is sufficiently high, a conducting channel may be formed where none is intended, e.g., a conducting channel between two adjacent MOS transistors which are meant to operate independently. The resulting parasitic transistor is called a field inversion transistor, and the process is called field inversion.
Field inversion is a particularly severe problem in N-channel MOS devices because the ratio between the threshold voltage causing undesirable field inversion and the threshold voltage causing the desirable inversion under the gate of a transistor is typically considerably lower than for p-channel devices. Various techniques have been used in the past in an effort to combat the field inversion problem in N-channel device. For example, the thickness of the field oxide has been increased as compared to the thickness of the gate oxide so as to raise the ratio between the field and gate threshold voltages. However, there are processing and cost limitations on how thick the field oxide can be made. The likihood of field inversion can also be reduced by lowering the gate voltage, but this decreases the performance of the device. It is also known that if the doping concentration of the substrate under the field oxide is sufficiently high, e.g., corresponding to less than 1 ohm centimeter resistivity, the field inversion properties of the resulting device would be adequate. In practice, however, the bulk substrate doping level can not be chosen this high because of body effect and mobility reduction, which result in degraded performance.
Based on knowing that high dopant concentration under the field oxide is desirable, it has been proposed to selectively dope the substrate, i.e., to have the bulk of the substrate at relatively low doping concentration and the portions of the substrate immediately below the field oxide at relatively high dopant concentration. One way to achieve selective doping of this type is by highly accurate ion-implantation. There is, however, one serious problem with such ion-implantation: in order to selectively ionimplant, it is necessary to mask the region where the implant is not wanted with a mask of photoresist, oxide or metal delineated with the desired pattern, and -- when the mask is removed -- it becomes very difficult to align the implant regions with the mask used in subsequent photomasking steps. It therefore becomes necessary to use extra masks merely for the purpose of providing registration marks. Another technique for selective doping for field inversion control is discussed in U.S. Pat. No. 3,751,722 and involves silicon nitride masking. It is well known, however, that processes using silicon nitride masking steps are less reliable and more difficult than processes using, for example, only silicon oxide masking. Thus, while it is known that selective doping can be useful for field inversion control, there is still a need for providing selective doping by a reliable and more practical process.